NB: hims056's answer is correct according to the law but I will add the following supporting information.

A lot is said in football about the referee's "interpretation" of the rules in a match. In the interpretation of this law, we should look closely at the term "immediately".

You will very rarely see a referee award a free kick against a player who passes or shoots with an un-booted foot. The player is at a disadvantage by being missing a boot so the generally-seen interpretation is that the player has had his shoe removed by accident.

With regards to the immediacy of the event, you're not likely to see a player running around for 15 minutes without a boot so pretty much any time from the loss of the boot up to the next time there is a break in the play could be deemed (interpreted) as immediate.

To sum up the interpretation as simply as possible; if the player has lost a boot and he hasn't had chance to replace it, then he will not have a foul awarded against him.